1. Field of the Invention
The present invention relates to a liquid crystal display device, and particularly relates to an in-plane switching mode (IPS mode) liquid crystal display device in which liquid crystal molecules are homogeneously oriented during black display, and an electrical field is applied to the molecules in a lateral direction, whereby light transmittance and blockage are controlled. The present invention is ideal for significantly improving the viewing-angle characteristics (particularly for black display and low gradation) of such a device.
2. Description of the Related Art
Liquid crystal display devices (also called “liquid crystal displays”) are basically composed of a liquid crystal (a layer of liquid crystal molecules, or liquid crystal layer) sandwiched between a pair of substrates. Among liquid crystal display devices of a system in which the direction of the electrical field applied to the liquid crystal is made parallel to the substrates (also referred to below as lateral electrical-field systems or IPS mode), systems employing a comb-tooth electrode provided on one substrate have been proposed in Japanese Examined Patent Application No. 63-21907, Japanese Laid-open Patent Application No. 9-80424, and Japanese Laid-open Patent Application No. 2001-056476. In these systems, the liquid crystal molecules primarily rotate in a plane parallel to the substrate due to the application of the electrical field. It is therefore well-known that the difference in the degree of birefringence when viewed diagonally between periods when an electrical field is applied and periods when a field is not applied is small, and the viewing angle is wide in these systems.
However, although changes in birefringence of the liquid crystal itself are small in IPS-mode liquid crystal display devices, it is known that, due to the characteristics of the polarizing plates layered on the substrates, light may leak out on viewing from a diagonal direction oriented away from the absorption axis of the polarizing plates. A method for using a retardation plate in order to stop such light leakage of polarizing plates in the diagonal direction has been disclosed in Japanese Laid-open Patent Application No. 2001-350022. However, the method disclosed in Japanese Laid-open Patent Application No. 2001-350022 basically improves the viewing angle of the polarizing plates alone. The effect of the liquid crystal was considered for VA mode, but a system for compensating for the effect of the liquid crystal layer in IPS mode is not disclosed in Japanese Laid-open Patent Application No. 2001-350022.
Disclosed in Japanese Laid-open Patent Application No. 2005-208356 is an invention for positioning a retardation plate to the inside of one of the polarizing plates in order to improve the viewing angle characteristics of black display. The invention disclosed in Japanese Laid-open Patent Application No. 2005-208356 also takes into account the effects of the supporting-substrate material of the polarizing plates and of coloring in the diagonal viewing angle during black display, and good performance can be expected. However, the necessary retardation plates are optically biaxially anisotropic media, positive C-plates (where nx=ny for the refractive index within a plane xy, and nz>nx for the refractive index nz in the thickness direction z), negative A-plates (where nx>ny, and nx=nz), or other materials having low productivity, and costs are therefore high.